Submarine aquifers Vast freshwater reserves are trapped beneath the ocean floor which could sustain future generations as current sources dwindle, say an international team of scientists.

In this week's issue of Nature they estimate 500,000 cubic kilometres of low-salinity water is buried beneath the seabed on continental shelves around the world, including off Australia, China, North America and South Africa.

"The volume of this water resource is a hundred times greater than the amount we've extracted from the Earth's sub-surface in the past century since 1900," says Australian lead author, Vincent Post, a groundwater hydrogeologist from Flinders University in Adelaide.

"It means that more options can be considered to help reduce the impact of droughts and continental water shortages."

UN Water, the United Nations' water agency, estimates that water use has been growing at more than twice the rate of population in the last century due to demands such as irrigated agriculture and meat production.

More than 40 per cent of the world's population already live in conditions of water scarcity. By 2030, UN Water estimates that 47 per cent of people will exist under high water stress.

Post says his team's findings were drawn from a review of seafloor water studies done for scientific or oil and gas exploration purposes.

Prior to this report Post says such undersea water reserves were considered to be rare.

"By combining all this information we've demonstrated that the freshwater below the seafloor is a common finding, and not some anomaly that only occurs under very special circumstances," he says.

Underwater aquifers

Post says the deposits were formed over hundreds of thousands of years in the past, when the sea level was much lower and areas now under the ocean were exposed to rainfall which was absorbed into the underlying water table.

When the polar icecaps started melting about 20,000 years ago these coastlines disappeared under water, but their aquifers often remained largely intact -- protected by layers of clay and sediment.

"In some case you have actually have fresh water under the sea, but in most cases it's a mixture between freshwater and sea water - we call that brackish water," says Post.

Post says the deposits are comparable with the bore basins currently relied upon by much of the world for drinking water and could cost much less than seawater to desalinate.

"For some areas it is economically viable to desalinate that brackish water and make it economically competitive with other sources of water recovery."

Post says similar technology to offshore oil exploration.

"You could drill from a platform or drill horizontally under the sea bed," says Post.

Challenges

But, Post says drilling for the water would be expensive, and Post says great care would have to be taken not to contaminate the aquifers.

He warned that they are a precious resource.

"We should use them carefully: once gone, they won't be replenished until the sea level drops again, which is not likely to happen for a very long time," says Post.

Water resources expert Dr Richard Davis of the Wentworth Group agrees.

While he says this "neglected resource" could become economically viable to obtain with the drop in desalination costs, humans don't have a good track record when it comes to exploiting the world's onshore groundwater resources, such as the Great Artesian Basin.

"We just squandered the water," says Davis, who was previously with CSIRO, the World Bank and the National Water Commission.

"In the Great Artesian Basin case we spent around 100 years pumping the water to the surface and letting it flow free and evaporate, using only a very very small fraction of what we tapped," he says.

As a result, says Davis, Australia has had to undertake a very expensive remedial program to try and cap the free-flowing bores to save water. He says there are similar stories in Africa and China.

"What concerns me here is that we don't take the same approach again," says Davis.

"Let's be slow, cautious and thoughtful about it this time and show how we can act responsibly if in fact the economics do stack up for the use of some of these aquifers."